Isononanoic acid, a mixture of structurally branched C9-monocarboxylic acids, is an important intermediate in industrial organic chemistry which is processed to give a multitude of conversion products for a wide variety of different fields of use. For example, the salts thereof are used as drying accelerators or siccatives for coatings, and the esters thereof with ethylene glycols serve as plasticizers for PVC or for polyvinyl butyral films and as coalescence agents in aqueous dispersions of polymers (Weissermel, Arpe, Industrielle Organische Chemie, VCH Verlagsgesellschaft, 3rd edition, 1988, p. 145; DE 10 2009 048 771 A1). The esterification of isononanoic acid with polyols such as neopentyl glycol, trimethylolpropane, ditrimethylolpropane, pentaerythritol or dipentaerythritol gives lubricant esters which are used in the operation of refrigerators. Isononanoic acid is frequently esterified in a mixture with other C4-C12-monocarboxylic acids such as 2-methylbutyric acid, n-pentanoic acid, n-heptanoic acid, 2-ethylhexanoic acid or n-octanoic acid. (EP 1 281 701 A1; EP 1 199 300 A2; EP 0 903 335 A1; WO90/12849 A1; EP 0 475 751 A1).
In addition, isononanoic acid is converted to the corresponding vinyl ester which, as a comonomer, modifies the properties of polymers such as polyvinyl acetate, polyvinyl chloride, polystyrene or polyacrylic esters. The corresponding copolymers can be processed to give paints which feature improved hydrolysis resistance and relatively low moisture absorption. Vinyl esters can be prepared by reaction of the isononanoic acids with acetylene, preferably in the presence of zinc salts at temperatures of 200-230° C. (G. Hübner, Fette, Seifen, Anstrichmittel 68, 290 (1966); Ullmann's Encyclopedia of Industrial Chemistry, 7th Edition, 2011, Wiley, Volume 38, pages 107-124; EP 1 057 525 A2), or by what is called the transvinylation reaction with an vinyl ester of another carboxylic acid, frequently vinyl acetate or vinyl propionate, in the presence of transition metal catalysts (Ullmann's Encyclopedia of Industrial Chemistry, 7th Edition, 2011, Wiley, Volume 38, pages 107-124; Adelmann, Journal Organic Chemistry, 1949, 14, pages 1057-1077; DE 199 08 320 A1, EP 0 497 340 A2, WO2011/139360 A1, WO2011/139361 A1).
The raw material used for the industrial preparation of isononanoic acid is the C4 cut from the steamcracking of naphtha. The availability thereof compared to the C2 and C3 cracking products can be controlled by the conditions of steamcracking and is guided by the market conditions.
1,3-Butadiene is first removed from the C4 cracking products by extraction or by selective hydrogenation to n-butenes. The resulting C4 raffinate, also called raffinate I, comprises predominantly the unsaturated butenes isobutene, 1-butene and 2-butene, and the hydrogenated products n-butane and isobutane. Isobutene is removed from the raffinate I in the next step, and the resulting isobutene-free C4 mixture is referred to as raffinate II.
For the isobutene removal, various processes are employed in industrial production, in which the highest reactivity of the isobutene in relative terms in the raffinate I is exploited. A known method is the reversible proton-catalysed addition of water to give tert-butanol, or methanol addition to give methyl tert-butyl ether. Isobutene can be recovered again from these addition products by redissociation (Weissermel, Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], VCH Verlagsgesellschaft, 3rd Edition, 1988, pages 74-79).
It is likewise possible to contact the butadiene-free C4 raffinate at elevated temperature and under pressure with an acidic suspended ion exchanger. Isobutene oligomerizes to diisobutene, triisobutene, and in a small portion to higher oligomers. The oligomers are separated from the unreacted C4 compounds. It is then possible to obtain diisobutene or triisobutene in pure form by distillation from the oligomer. The dimerization of n-butenes with isobutene forms co-dimer to a small degree (Weissermel, Arpe, Industrielle Organische Chemie, VCH Verlagsgesellschaft, 3rd Edition, 1988, p. 77; Hydrocarbon Processing, April 1973, pages 171-173).
Diisobutene, either prepared by the oligomerization of pure isobutene obtained by redissociation or obtained in the course of workup of a butadiene-free raffinate I, is then converted to a C9 derivative lengthened by one carbon atom. Industrial operation involves the hydroformylation or oxo process in which diisobutene is converted to the corresponding aldehyde with carbon monoxide and hydrogen in the presence of rhodium or cobalt catalysts. Since diisobutene predominantly comprises the octenes 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, the hydroformylation reaction gives the C9 aldehyde 3,5,5-trimethylhexanal as the main constituent. Further C9 isomers present in small amounts are 3,4,4- and 3,4,5-trimethylhexanal, and also 2,5,5-trimethylhexanal, 4,5,5-trimethylhexanal and 6,6-dimethylheptanal. Oxidation of this aldehyde mixture gives an industrially available isononanoic acid typically having a content of 3,5,5-trimethylhexanoic acid of about 90% (Ullmanns Encyklopädie der technischen Chemie, 4th Edition, 1975, Verlag Chemie, Volume 9, pages 143-145; EP 1 854 778 A1).
Diisobutene can likewise be converted by what is called the hydrocarboxylation or Koch reaction with carbon monoxide and water in the presence of sulphuric acid to the highly branched isononanoic acid 2,2,4,4-tetramethyl-1-pentanoic acid. Owing to the double alkyl branch at the carbon atom adjacent to the carboxyl group, this isononanoic acid is frequently also referred to as neononanoic acid (Ullmanns Encyklopädie der technischen Chemie, 4th Edition, 1975, Verlag Chemie, Volume 9, pages 143-145).
The n-butenes present in raffinate II after the isobutene removal are also converted industrially to butene oligomers, from which isomeric octenes are separated, and these are converted via the hydrocarboxylation to the corresponding isononanoic acids (DE 199 08 320 A1; DE 199 06 518 A1). The oligomerization of n-butenes is conducted industrially over acidic catalysts such as zeolites or phosphoric acid on supports. This gives octenes comprising dimethylhexenes as the main product. Further processes include the DIMERSOL process and the OCTOL process. The DIMERSOL process works with soluble nickel complex catalysts and leads to an octene mixture having a high proportion of 3- and 5-methylheptenes, in addition to dimethylhexenes and n-octenes. In the OCTOL process, supported fixed bed nickel catalysts are used and the resulting octene mixture features a low degree of branching (DE 199 08 320 A1, WO 03/029180 A1, Hydrocarbon Processing, February 1986, pages 31-33). According to DE 199 08 320 A1, the respective differently branched octene mixtures are converted via the hydrocarboxylation to the corresponding isononanoic acids, which are then converted to the corresponding vinyl esters. Vinyl esters of isononanoic acids based on an octene mixture from the OCTOL process are suitable as a plasticizing comonomer.
Against the background that the availability of octenes based on the C4 cut from naphtha cracking is limited and depends on the local conditions, it is desirable to develop further octene sources based on inexpensively available large-scale products which can be transported to various sites in a simple manner.
2-Ethylhexanol is available inexpensively as an industrial large-scale product which can be sold widely without any problems. As is well known, 2-ethylhexanol is prepared on the industrial scale by hydroformylation or oxo process using propylene to give n-butyraldehyde with subsequent alkali-catalysed aldol condensation to give 2-ethylhexenal followed by full hydrogenation to give 2-ethylhexanol (Ullmann's Encyclopedia of Industrial Chemistry, 7th Edition, 2011, Wiley, Volume 13, pages 579-584).
WO 03/029180 A1 briefly discusses the use of 2-ethylhexanol for preparation of an octene mixture which is processed via dehydration, hydroformylation and hydrogenation to give an isononanol mixture. The emphasis is on the adjustment of the viscosity of the isomeric dialkyl phthalates which are obtained by esterification of isomeric nonanols with phthalic acid or phthalic anhydride. No pointers are given to convert the dehydration products of 2-ethylhexanol to isononanoic acid.
The utilization of 2-ethylhexanol as the octene source enables the provision of isononanoic acid based on propylene, and reduces dependence on octene availability based on butene.